Contact image sensors (CISs) have recently been developed and put into practical use along with reductions in size and weight in a scanner apparatus for inputting an image to a personal computer and the document reader of a facsimile apparatus. The contact image sensor is formed from a light source, rod lenses which implement a ×1 erect optical system, and a one-dimensional light-receiving sensor whose effective width is substantially the same as the maximum width of a document to be read. The CIS can shorten the optical path length of an optical system, compared to an optical system which uses a lens called a reduction optical system to reduce the main scanning width of a document to be read to a fraction of the original width and reads the image by a one-dimensional CCD sensor or the like. Accordingly, the CIS can implement a small-size, lightweight document reader.
With the recent advance in semiconductor process and production technique, CISs have been adopted in A4-size consumer scanners. With low cost and a small light quantity from a light source, the use of CISs in high-speed image reading apparatuses such as a copying machine is also being examined.
A CIS requires a length equal to the width of a document image in reading an image. Forming a sensor with this length from one sensor chip is less advantageous in terms of the technique and cost. The CIS generally ensures a reading length equal to the document image width by arraying a plurality of sensor chips.
In a copying machine using a CCD image sensor, a CCD image sensor which is divided into two blocks at the center and reads out signals from the two ends in order to cope with a high-speed copying machine has been studied.
In the CIS, the plurality of sensor chips are linearly arrayed, as described above. Image signals from the sensor chips undergo well-known shading correction to adjust their black and white levels. The white level is generally adjusted by pixel, and the black level is adjusted by chip, pixel, or the like, which are both known well. The known shading correction is a reading correction method which assumes an ideal linearity of a read image signal. If the linearity characteristics of the chips of a multi-chip vary, the image signal level also varies at the halftone level. The density is different between adjacent chips, seriously degrading the quality.
Even if the black and white levels are adjusted by known shading correction in the CCD image sensor which is divided into two blocks at the center and reads out signals from the two ends, the linearity characteristic varies between the right and left sides due to differences in read characteristics at the two ends and the characteristics of a plurality of amplifiers. The levels of read signals differ from each other at the halftone level, resulting in a density difference and serious degradation in image quality. The density difference and quality degradation are conspicuous at the central boundary.
In general, these problems of the above-mentioned CCD image sensor have been solved by performing linearity correction using an LUT (Look Up Table) stored in a known ROM or RAM.
An LUT can realize arbitrary signal conversion by supplying an input signal as an address and reading out data stored at the address as an output signal. When the number of signals to be corrected is small, the correction method by signal conversion using the LUT is an ideal correction method.
However, when the correction method using the LUT is directly used in a color CIS with an array of 16 sensor chips, 16 chip×3 colors=a total of 48 LUTs are required. This leads to an impractically large-scale circuit regardless of whether the LUTs are constituted by discrete memories or incorporated in an ASIC.
When the conventional ODD/EVEN output (i.e., odd- and even-numbered pixels are read using different reading systems) is executed from two ends in a CCD image sensor which reads out signals from two ends, a monochrome image requires four LUTs, and a color image requires 12 LUTs, which is three times the number of LUTs for a monochrome image. A large number of LUTs cannot be ignored, either.
In short, output variations of a plurality of systems with different linearity characteristics that cannot be completely corrected by known shading correction, e.g., halftone linearity variations of a plurality of chips, blocks, or output systems with different characteristics are closed up as a result of implementing the multi-chip of a CIS or multi-read of a high-speed CCD. Especially for a color image, difficulty lies three times.
At present, the reader of a copying machine or the like is normally OFF for power saving and low power consumption. Even if the user wants to activate the copying machine and read a document as quickly as possible, a time of about 5 sec is taken for calculating LUT curves and writing the curves to RAM by the CPU even in the use of the simplest straight line in an arrangement where LUTs are prepared for the number of chips, blocks, or output systems subjected to linearity correction. Also from this viewpoint, the use of many LUTs is undesirable.